1. Field of the Invention
This invention relates to improvements in or relating to an electrical exposure control device for cameras of the so-called through-the-lens-type light measuring system for measuring the brightness of an object to be photographed through a photographing lens of the camera.
2. Description of the Prior Art
Hitherto, an electrical exposure control device for the camera of a through-the-lens-type light measuring system in which an output of its light measuring circuit derived immediately before the operation of a shutter is memorized in a digital form during the shutter operation. As a result, the exposure time is automatically controlled in accordance with the memorized value, has been proposed in the art, as disclosed, for example, in the Japanese Pat. Publication No. 4930/1970. Namely, in the device set forth in the above-identified Patent Publication, a pulse generator circuit provides a pulse whose frequency varies with the brightness of an object to be photographed. The pulse is counted by a counter for a certain period of time; the brightness of the object is converted into a digital value and stored; and the exposure time is controlled in accordance with the stored content. However, it is extremely difficult to obtain a pulse generator circuit whose frequency linearly varies over a wide range of radiation intensity values, and consequently the aforesaid device is not practical. Further, there has also been proposed such a device which employs a pulse generator circuit of a constant frequency and in which a pulse derived therefrom is applied to a time constant circuit consisting of a photoelectric element and a capacitor; the time for the capacitor to be charged up to a predetermined level is counted and the counted content is utilized for determining the exposure time (refer to, for example, U.S. Pat. No. 3,703,130 or Deutch OS 2,114,525). With this device, the time period for measuring the quantity of light varies with the brightness of an object to be photographed and becomes long especially where the object is dimly illuminated. Thus, there arises various problems in the construction and operation of such a device. In both of the above devices, where a flip-flop circuit is used as the counter, it is necessary to employ at least the same number of flip-flop circuits as the stages for setting the exposure time. This introduces complexity in the circuit construction and requires many switches for actuating the circuits, which also results in complicated circuit construction.
Further, the indication of the exposure time of the automatic exposure setting provided for the camera operator is achieved by a meter and is confirmed by the position of a pointer of the meter whether the brightness of the object to be photographed falls inside or outside of a photographic range. However, the indication by such an indicator is an analog one, so that the recognition of the information being indicated may be difficult. In addition, the meter does not readily withstand mechanical shocks and hence, is likely to be broken. Moreover, an indicator is known for indicating the exposure time by a meter and for giving warning by turning on and off a lamp or the like when the brightness of the object is outside of the characteristic range in which a light receiving element is capable of measuring the light. However, this indicator requires two kinds of indicating sections, and there are problems of recognizing these separate indications, and of the susceptibility to the influence of mechanical shocks.